Multiple-level actuators and clamping devices

ABSTRACT

Improved fabrication processes for microelectromechanical structures, and unique structures fabricated by the improved processes are disclosed. In its simplest form, the fabrication process is a modification of the know SCREAM process, extended and used in such a way as to produce a combined vertical etch and release RIE process, which may be referred to as a “combination etch”. Fabrication of a single-level micromechanical structure using the process of the present invention includes a novel dry etching process to shape and release suspended single crystal silicon elements, the process combining vertical silicon reactive ion etching (Si-RIE) and release etches to eliminate the need to deposit and pattern silicon dioxide mask layers on the sides of suspended structures and to reduce the mechanical stresses in suspended structures caused by deposited silicon dioxide films.

This application claims the benefit of Provisional Application No.60/256,358, filed Dec. 19, 2000, the disclosure of which is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to a novel process for thefabrication of suspended high-aspect-ratio single crystal siliconmicrostructures. More particularly, the invention relates to two-,three- or more level, high-aspect-ratio, released, or suspended, singlecrystal silicon microstructures of complex geometries, wherein alllevels are self-aligned and fabricated from a single silicon wafer, andto the isolation of or removal of selected suspended microstructures.

Interest in the fabrication of suspended microstructures has increasedwith the increased use of microelectromechanical systems (MEMS). ManyMEMS implementations are fabricated by surface micromachining ofdeposited thin films, by bulk micromachining of (usually silicon)substrates, or by deep etching, as used in the Single Crystal ReactiveEtching and Metallization (SCREAM) process, which is explained forexample, in U.S. Pat. No. 5,198,390. As described in this patent, theSCREAM process produces a single level of suspended microstructures.Multiple level SCREAM microstructures must use multiple substratesbonded together, and most other prior techniques for fabricatingmultiple level structures require the assembly and alignment of numerousseparately fabricated components. However, such assemblies areimpractical for large arrays of micron-scale structures, and inparticular for electron lenses and similar devices, where alignment iscritical.

Another process for micromachining silicon substrates for fabricatingmicroelectromechanical devices is exemplified by the process describedin U.S. Pat. No. 5,501,893 to Laermer, et al. In this patent, ananisotropic plasma etching of silicon through a mask is used to providelaterally defined recesses in a substrate. The etching step isalternated with a polymerizing step which covers silicon surfacesexposed by the etching step to provide an etching stop, which preventsetching of those surfaces in a subsequent etch step. The alternatingetching and protecting steps allow high anisotropy of the etchedstructure.

Although these processes have been successful, new technical demandshave emerged for MEMS fabrication technology; in particular, theintegration of MEMS with active electronics on the same chip, and thedevelopment of more complex microsystems. There is a need, therefore,for a process which will facilitate fabrication of single and multiplelevels of released or suspended microstructures from a single substrate,and in particular will facilitate the fabrication of large arrays ofthese structures.

SUMMARY OF THE INVENTION

Briefly, the present invention is directed to improved fabricationprocesses for microelectromechanical structures, and to uniquestructures fabricated by the improved processes. In its simplest form,the invention is directed to a fabrication process which is based onboth the method described, for example, in U.S. Pat. No. 5,501,893 toLaermer et al (hereafter Laermer et al) the disclosure of which ishereby incorporated herein by reference, and on the SCREAM processdescribed, for example, in U.S. Pat. No. 5,198,390, the disclosure ofwhich is hereby incorporated herein by reference. These processes aremodified and extended and are used in such a way as to produce acombined vertical etch and release RIE process, which may be referred toherein as a “combination etch”.

Fabrication of a single-level micromechanical structure using theprocess of the present invention includes a novel dry etching process toshape and release suspended single crystal silicon elements, in a singledry etch step. This new process combines vertical silicon reactive ionetching (Si-RIE) and release etches to eliminate the need to deposit andpattern silicon dioxide mask layers on the sides of suspendedstructures, as is required in the SCREAM process. It furthermore reducesthe mechanical stresses in suspended structures caused by depositedsilicon dioxide films.

Briefly, the combined process includes formation of a mask structure onthe surface of a silicon substrate, for example, through conventionalphotolithography. Thereafter, the silicon is etched through the maskusing an anisotropic vertical reactive ion etching (RIE) plasma etchingprocess using a sequence of etch and passivation cycles as described byLaermer et al. A short quasi-isotropic Si-RIE is followed by apolymerization step which conformally coats the exposed surfaces with apassivation layer. Thereafter, a second anisotropic etching step iscarried out which first removes the polymer coating from the horizontalsurfaces of the substrate by sputter etching, but not from the verticalsurfaces, and then etches the exposed silicon quasi-isotropically. Thesealternating steps are repeated to produce deep structures (or trenches)having vertical edges, with the profile of the structure beingdetermined by the balance between the passivation and etching steps.

When the desired structure depth has been reached, the structure may bereleased from the substrate by adjusting the length of and balancebetween the passivation and etching steps. A long polymerizationdeposits a thicker passivation layer, followed by a longer Si-RIE stepwhich undercuts the structure to release it. This process integrates therelease of suspended structures with the vertical etch that definesthem, and is much simpler than prior deep etching fabrication processsequences. It allows the fabrication of delicate structures by reducingmechanical stresses in them, due to the absence of stress-inducingsidewall passivation layers and by avoiding any wet process steps whichcould deform the suspended structures. The process is particularlysuitable for integration with active sensing devices.

In accordance with this aspect of the invention, silicon suspendedstructures may be produced on a single crystal silicon (SCS) substrate,or wafer, which may contain prefabricated active devices, utilizing onlyfour steps: (1) a photolithography step which defines the layout of thesuspended structures and surrounding trenches in a resist layer on thesubstrate; (2) a first etching step in which the pattern is transferredto the field and inter-layer dielectrics are etched in an anisotropicRIE step; (3) a second etching step in which high aspect ratio SCSsuspended structures are etched and released in a single combined etch,and (4) a final step in which the photoresist is removed. The sidewallsilicon dioxide film deposition and etchback utilized in prior processessuch as the SCREAM process is not necessary, and no further patterningsteps for interconnecting the released structure to active devices onthe wafer are required, in accordance with this process. On a blanksilicon wafer without dielectric films step (2) can also be skipped,reducing the number of process steps for the fabrication of suspendedsingle crystal silicon structures to 3.

The process of the present invention further comprises an extension ofthe SCREAM process to allow the fabrication of multiple levelstructures, including but not limited to, the selective removal of partsof levels. This extension of the process enables the fabrication ofsuspended elements of different heights, and reduces the process-imposedrestriction of previous multiple level processes that all suspendedlevels must have the same layout. The extended process is applied to thefabrication of novel actuators, electron lenses and micromachines, andallows multiple levels of self-aligned, suspended structures withgreater design flexibility.

In the basic SCREAM process, as described in U.S. Pat. No. 5,198,390, athick silicon dioxide film (1.5-4 μm) is placed on the surface of asubstrate by thermal oxidation of the substrate or by CVD deposition.This film is patterned using a resist layer, photolithography, andCHF₃-RIE. The silicon substrate is etched in a vertical RIE step, usingthe silicon dioxide and photoresist films as an etch mask. Typical etchdepths range from 10 to 20 μm. A subsequent thermal oxidation or CVDdeposition covers the surface conformally with a thin (200-400 nm)silicon dioxide layer. A vertical RIE then removes this film from thehorizontal surfaces, including the floor of trenches etched by the firstvertical RIE; any silicon dioxide on the sidewalls of the SCS structureis not etched.

The top of the SCS structure is still covered with the remainder of thefirst, thicker silicon dioxide etch mask. A high-pressure (80-90 mTorr)isotropic SF₆ RIE of silicon then undercuts the masked siliconstructure, completing the fabrication of the first level of structures.In a final metallization step aluminium is deposited by sputtering,covering the released structures nearly conformally.

A multiple level structure can be fabricated using an extension of theSCREAM process, with the same four steps being repeated for each levelof the structure: vertical Si-RIE, conformal silicon dioxide deposition,anistropic CHF3 RIE of silicon dioxide and quasi-isotropic RIE ofsilicon using SF6. After carrying out these steps for the first level ofreleased silicon structures, the silicon substrate is again etched in avertical RIE for a further 10-20 μm, without the need for an additionallithographic step; instead, the whole upper-level masked siliconstructure serves as a ‘shadow mask’ for this etch. The exposed siliconunderneath the masked silicon structure is not etched due to the highdegree of anisotropy of the vertical Si-RIE. The resulting lower-levelsilicon structure is self-aligned to the upper-level structure, but isslightly wider than the original width of the upper level, since thesidewall oxide deposition increased the width of the upper levelstructure. A thermal oxidation or CVD deposition again covers allsurfaces conformally with silicon dioxide.

A vertical CHF₃ RIE again is used to remove the oxide from thehorizontal surfaces, with the top of the upper level of siliconstructure still being covered by the remaining part of the first (thick)mask oxide. An isotropic SF₆ RIE undercuts and releases the maskedsilicon structure to complete the fabrication of the second level of themicroelectromechanical (MEM) structures. This sequence is repeated foreach additional level.

In accordance with the present invention, fabrication of multileveldevices may be further simplified by using the combined etch processbased on the SCREAM process and the etch process of Laermer et al. Thecombined etch is used to produce the first layer of a structure, and theSCREAM process is utilized for second and subsequent layers. Thiscombination simplifies the fabrication of multilevel structures byomitting several of the silicon dioxide deposition and etchback stepsthat would be required if only the SCREAM process were to be used. Themodified Laermer et al etch could be used for just one layer or foradditional layers, but is not preferred for lower level structures dueto the increased impact of its limitations; namely, the maximumlinewidths which can be released and the decreasing effectiveness of therelease etch in deep, narrow trenches.

After the completion of all levels of the suspended microstructure, atimed thermal oxidation step is used to isolate selected suspended SCSstructures by consuming all the silicon in thin isolation segments andto increase the protection of the released structures during subsequentprocessing.

The structures are then covered with a thick photoresist film (typically20-60 μm) so a second lithography step may be used to open contactwindows on the suspended structures. These windows are designed to belarge enough (>20 μm features) to be easily exposed in the thick resistand do not require critical alignment to the microelectromechanicalbeams (which typically are 0.6 to 4 μm wide). Moreover, they do not haveto be exposed through the full thickness of the resist but only down tothe top of the upper level of the beams making up the isolationstructures, when the contacts are to be made on the top surfaces. Theresist forms an etch mask for a wet buffered oxide etch which may beused to open up the contact windows. This etch may also used toselectively remove the SiO₂ films from parts of the suspended structurein which case The openings in the resist etch mask have to be largeenough to enable the underlying structures to be exposed to the wetetch. After stripping the resist, contacts to the suspended siliconstructures are formed by the evaporation of aluminum, followed by asintering step.

The isolation and contact scheme for multiple-level self-alignedstructures selectively contacts and isolates levels, individually aswell as all of them simultaneously, while maintaining mechanical supportof the suspended structures. Furthermore, the scheme provides a way toelectrically connect to the suspended levels from the outside viacontact pads through a combination of five elements, which provide (I)electrical isolation on all levels, but mechanical support from thesubstrate, (ii) electrical isolation within the suspended levels, (iii)isolation on one level while connecting the others, (iv) electricalconnections between levels, and (v) electrical connection from anarbitrary level to the top of a contact pad. For practical reasons thesepads are preferably at the wafer surface for all levels.

The multiple-level contact and isolation scheme of the invention makesuse of the feature of the present process that the lower levels arewider than the upper levels, as described above, which widening iscaused by the broadening of the structures in the sidewall oxidedeposition step. As noted above, the upper level is etched by a verticalSi-RIE, which is followed by a thermal oxidation of the sidewalls. Thisoxidation step increases the width of the structures even though some ofthe silicon is consumed; the volume of the resulting silicon dioxide is2.2 times larger, which leads to a net widening of the structures by anamount roughly equal to the film thickness. Alternatively, the sidewallfilm may be deposited by CVD, in which case the widening is twice thedeposited film thickness. This widened structure now forms the mask forthe vertical silicon RIE which defines the lower level structure. Thislower level structure is thus wider than the upper level by about thesidewall film thickness. Both levels are again thermally oxidized andthen the lower level is released as well. This may be followed by a longoxidation step, designed to consume all the silicon in the thinnestsegments of the released beams to produce silicon dioxide isolatingsegments in the beams.

The strength of this isolation scheme is the simplicity of the processsequence, for only one lithography step and one metal deposition isrequired, independent of the number of levels. Thermal silicon dioxidebridges provide high quality insulation between suspended levels and thesubstrate with low leakage currents and high breakdown voltages.

The present invention, in another aspect, comprises new types ofactuators using multiple levels of electrodes which preferably arefabricated using the improved process of the invention. These actuatorsshow an increased range of motion compared to conventional single-levelelectrostatic comb actuators and generate higher forces. They includedesigns which operate bi-directionally and allow multistableconfigurations. As these new actuator designs generate a higher forceper substrate area used, they are of particular interest for theintegration of dense arrays of actuators. In order to effectively usethe advantages of the multiple-level designs, the gaps between theelectrodes need to be kept small, and the electrodes accurately aligned.

In still another aspect, the present invention comprises a new type ofclamping device, which allows the accurate positioning and clamping ofmicro- to mini-scale elements, such as optical fibers, by exploiting theself-aligned nature of the multiple levels.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features and advantages of thepresent invention will become more clearly understood from the followingdetailed description of preferred embodiments thereof, taken with theaccompanying drawings, in which:

FIGS. 1a to 1 h diagrammatically illustrate the prior art SCREAMprocess;

FIG. 2 is a diagrammatic illustration of a single-level MEM structurefabricated by the process of FIGS. 1a-1 h;

FIGS. 3a-3 e diagrammatically illustrate a first embodiment of thepresent invention;

FIGS. 4a-4 j diagrammatically illustrate a second embodiment, which is amultiple-level extension of the process of FIGS. 1a to 1 h, providingoxidation and removal of upper level(s);

FIG. 5 is a diagrammatic enlarged cross-sectional view of a two-levelMEM structure fabricated in accordance with the process of FIGS. 4a-4 h;

FIG. 6a is a diagrammatic cross-sectional view of a two-level MEMstructure fabricated in accordance with the process of the invention toelectrically isolate both suspended levels from the substrate and toprovide an electrical contact to the upper level;

FIG. 6b is a photomicrograph of the two-level MEM structure illustratedin FIG. 6a;

FIG. 7a is a diagrammatic enlarged cross-sectional view of amodification of the two-level MEM structure of FIG. 6a which is used toelectrically isolate both suspended levels from the substrate and toprovide an electrical contact to the lower level, and which includes theselective removal of part of the upper level structure;

FIG. 7b is a photomicrograph of the two-level structure illustrated inFIG. 7a;

FIG. 8 is a diagrammatic illustration of isolation schemes for the MEMstructures of the invention, in this case for 3 suspended levels;

FIGS. 9a-9 c are diagrammatic illustrations, in cross-section, ofcurrent paths for 3-level MEM structures;

FIGS. 10 and 11 illustrate design variations for a two-level structure;FIGS. 12 and 13 illustrate design variations for a 3-level structure;FIGS. 14a and 14 b illustrate another multiple-level extension of theprocess of the invention, including focused ion beam modification ofsuspended structures illustrating the elimination of an upper level;

FIGS. 15a and 15 b illustrate focused ion beam modification of lowerlevel element(s);

FIG. 16 illustrates the geometrical limitations of the technique ofFIGS. 14 and 15;

FIGS. 17a-17 e illustrate multiple level and multiple height actuatorsfabricated in accordance with the process of the present invention;

FIG. 18 illustrates force vs. displacement for multiple-height actuatorswith the moving electrode grounded;

FIG. 19 is an enlarged view of a portion of the graph of FIG. 18;

FIG. 20 illustrates force vs. displacement for multiple-height actuatorswith the moving electrode biased;

FIG. 21 illustrates the dependence of vertical force on substrate floorto structure separation;

FIGS. 22 and 23 illustrate the bi-directional operation of a firstembodiment of a multi-level actuator, showing normalized force vs.displacement for both levitation (solid line) and pull-in (dashed line)operation;

FIGS. 24 and 25 illustrate bi-directional operation of a secondembodiment of a multi-level actuator, showing normalized force vs.displacement for both levitation (solid line) and pull-in (dashed line)operation;

FIG. 26 illustrates a comparison of multiple-level actuator designs,showing normalized force vs. displacement;

FIG. 27 illustrates a comparison of push- and pull-modes of thebi-directional actuator of FIG. 22;

FIGS. 28a and 28 b illustrate normalized force vs. displacement for atwo-level bistable system;

FIG. 29 illustrates potential energy vs. displacement for aspring-actuator system, the dotted line representing no applied force;

FIGS. 30a and 30 b are diagrammatic top plan views of a multiple-levelfiber Clamp-Alignment device, FIG. 30a showing a self-aligned initialposition with no displacement of levels and FIG. 30b showing therelative displacement of upper and lower levels and the resultantalignment of a device being clamped; and

FIGS. 31a and 31 b are diagrammatic illustrations of the lateralinstability of comb-drive actuators, FIG. 31a showing a moving fingeraligned at center and FIG. 31b showing a moving finger off-center.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to a more detailed description of the processes used in thepresent invention, FIGS. 1a-1 h illustrate in diagrammatic form theprior art Single Crystal Reactive Etch and Metallization (SCREAM)process for fabrication of microelectromechanical MEM) structures anddevices, as described in greater detail in U.S. Pat. No. 5,198,390.Broadly, this process includes the steps of silicon mask deposition(FIG. 1a); patterning of the mask layer by photolithography and reactiveion etching (RIE)(FIG. 1b); deep vertical silicon etching by Si-RIE(FIG. 1c); conformal deposition of silicon dioxide (FIG. 1d);anisotropic etchback of the silicon dioxide (FIG. 1e); an optionalsecond vertical Si-RIE which is an extension etch (FIG. 1f); anisotropic RIE of silicon, as a release etch (FIG. 1g); and an aluminumsputter deposition (FIG. 1h).

In greater detail, the SCREAM process begins with the deposition of asilicon dioxide layer 10, typically 1-3 μm thick on a wafer or substrate12. Next, photolithography on a resist layer 14 is used to define thelayout of the MEM structure and the pattern is transferred by RIE to thesilicon dioxide layer to form a mask 16. Usually this is done bymagnetron ion etching (MIE) which achieves higher etch rates, forexample 250-300 nm/min, than conventional parallel-plate RIE (<30nm/min). The patterned silicon dioxide layer 10 and resist layer 14serves as the etch mask 16 for a subsequent vertical RIE of the siliconsubstrate, which forms trenches 18 typically to a depth between 10 and30 μm, as illustrated in FIG. 1c. After stripping the resist 14, thewafer is coated conformally with 100-300 nm of silicon dioxide 20 in aCVD deposition step. The silicon dioxide 20 is then etched back onhorizontal surfaces in an anisotropic RIE, exposing the siliconsubstrate on the floors 22 of the trenches 18. The sidewalls 24 of theetched silicon structures are still covered by the oxide film 20, whilethe tops of the structures are covered by the remainder of the (thicker)mask oxide 10 deposited in the first process step.

An optional second vertical RIE of the silicon substrate is used toextend the etched trenches 18 further into the substrate, as indicatedat 26 in FIG. 1f, typically by between 5 to 10 μm. At this point in theprocess, the upper parts of the trench sidewalls 24 are covered by asilicon dioxide film, while on the lower parts 28 of the sidewalls thesilicon is exposed. An isotropic RIE of silicon using SF₆ gas and highpressure (80-100 mTorr) is then used to undercut the SCREAM structuresin regions 28 to remove the exposed silicon in the lower part, butsilicon will not be removed in the upper part since it is protected bythe silicon dioxide mask 20. There is some etching up into the siliconstructure underneath the oxide mask 20, as at 30 and 32, generating anoverhanging silicon dioxide film or ‘skirt’ 34 at the bottoms of theresulting dioxide-covered SCS structures 36 and 38.

In the final process step, a metal layer 40, such as aluminum (about 300nm) is deposited by sputtering, as illustrated in FIG. 1h. The metalcoverage is nearly conformal; the horizontal and vertical surfaces areall coated with aluminum, but the undercut regions 30 and 32, under thesilicon dioxide ‘skirt’ are not covered. Thus, the deposited aluminumfilm does not contact the exposed part of the silicon core of thesilicon structures, or beams, from underneath, and is elsewhere isolatedfrom the core by the silicon dioxide layer.

FIG. 2 is a diagrammatic illustration of a simple MEMS device 40fabricated on a substrate 42 using the SCREAM process. The entirestructure is covered with aluminum, including the spring supports 44 and46 which suspend the structure in a trench 48 in the substrate. The Alfilm on the spring provides the electrical connection of comb capacitorelectrodes 50 to contact pads 52 on top of anchors 54. The isolation ofthe Al-film is maintained as well for the fixed structures and anchorsfrom which the beams are suspended as long as they are surrounded by atrench. The anchor is undercut by the isotropic release etch, but notcompletely like the suspended beams, so it is still connected to thesubstrate. It does however have the same oxide skirt which isolates theAl-film on the top and sides of the structure from the siliconunderneath.

A wide variety of SCREAM MEMS have been fabricated, includingaccelerometers, friction and microloading machines, and tunableresonators. Typical SCREAM beam dimensions are a silicon height of 10-30μm and a silicon width of 1 to 2 μm to produce a high aspect ratio beam,a remaining top oxide film thickness of about 0.5 to 1 μm, 200-400 nmthickness of sidewall oxide and 300 nm thickness of Al. The high aspectratio of the beams makes them very stiff with regard to out of planemotion and allows them to span lengths of up to several millimeters.

The SCREAM process described above requires only a single lithographystep and uses a dry release step which eliminates stiction problemscommonly observed during the wet release etch in surface micromachining.It uses only low-temperature (<300° C.) steps and can thus be performedon wafers with prefabricated electronic circuits. However, thesimplicity of the SCREAM process also imposes some limitations on theresulting devices, which are overcome by the process of the presentinvention.

The Laermer et al process, as modified in accordance with the presentinvention to yield a combined vertical Si RIE and release etch, isillustrated in FIGS. 3a to 3 e, to which reference is now made.Following conventional lithography to produce a mask 60 on a siliconsubstrate 62, a short, nearly isotropic plasma RIE silicon etch,indicated by arrow 64 in FIG. 3a, etches the exposed surface 66 of thesilicon to form a shallow trench 68. The etch undercuts the mask 60slightly, as illustrated at 70.

Following the first etch step, a first passivation layer 72 is appliedconformally to all exposed surfaces, including the exposed siliconsurface 66 and the sidewall silicon surface at undercut region 70 belowthe mask 60, as illustrated in FIG. 3b. This layer 72 is preferably apolymer film, and forms an effective etching stop layer on the sidewalls.

A second, nearly isotropic Si-RIE etching step, illustrated in FIG. 3cat arrow 74, breaks through the polymer layer 72 on the horizontalsurfaces of the trenches 68 by ion bombardment (sputter etching) anddeepens the trenches as indicated at 76 by etching the exposed silicon.During this second step, the side walls of the structures 78 and 80being etched remain protected by the polymer layer. The second etchproduces additional undercuts 82 and 84 beneath the protective polymerlayer applied in the previous step. A second passivation layer is nextapplied to the exposed silicon sidewall and floor surfaces, and theprocess is repeated, as illustrated in FIG. 3d, to produce a desiredtrench depth and resulting structure height and width. The sidewallprofile of the etched silicon structures is determined by the balancebetween etching and passivation steps.

In accordance with the present invention, an additional step is utilizedin combination with the foregoing process to effect release of etchedstructures without the additional sidewall passivation layers requiredin the SCREAM or similar processes. A long passivation step is used todeposit a thicker polymer film as indicated for structure 78 in FIG. 3e.In order to release the structure, a long quasi-isotropic Si RIE,indicated by arrows 90, is used. This etch removes the polymer from thehorizontal surfaces by sputter etching and deepens trench 76, and alsoetches the sidewalls 82 on structure 78 and the sidewall 82 on structure78 and the sidewall 84 on structure 80 to undercut these structures at92 and 94, respectively. The width of structure 78 is selected, duringthe masking process, to be such that the undercut 92 extends throughstructure 78 to release it, whereas structure 80 is undercut, but notreleased.

The width of structures which can be released is limited by the abilityof the sidewall passivation to protect the structure created in thepreceding vertical etch. This requires careful selection of the releaseetch rate and isotropy, and of the amount of passivation required tosustain the release. The release timing is determined by threeparameters: the lateral etch rate of silicon, and the lateral etch rateand the vertical etch rate of the passivation film during the releasestep. The lateral silicon etch rate determines the etch time required torelease the structures, the vertical etch rate of the passivation filmdetermines the time required to etch the passivation on the floor, andthe lateral etch rate of the passivation determines the maximum etchtime before the released silicon structures themselves are damagedduring the release etch.

In tests, it was found that for the parameters used, the passivationfilm on horizontal surfaces is etched approximately five times fasterthan it is deposited, reducing the effective etch time by 20% of thedeposition step duration. The lateral silicon etch or release rate wasfound to be approximately 1.6 micron/min. There was a slight deviationfrom the linear relationship at higher etch and deposition times, whichwas attributed to reduced deposition rate with increased film thickness.In order to guarantee sufficient sidewall protection it was found thatthe ratio of etch to deposition time should be kept at or below 1.2. Foruse in a multiple level process, the etch time should be minimized tominimize the vertical gap between the released structures and thesubstrate floor. This is important to obtain efficient shadowing duringsubsequent anisotropic silicon and oxide etches. Furthermore, thecombined etch is more difficult to use for lower level structures due tothe reduced etch rates, especially in deep, narrow trenches.

In accordance with this invention, multiple level MEM devices can befabricated using either the SCREAM process or the Laermer et al processfor the first level, and then using the SCREAM process for subsequentformation of lower levels. The SCREAM/SCREAM process is diagrammaticallyillustrated for two-level structures in FIGS. 4a-4 j, to which referenceis now made. It will be understood that these Figures show only a smallportion of a MEMS device in cross-section to demonstrate the processsteps. Broadly, the process includes depositing a silicon dioxide filmon a substrate and patterning the film using conventional resist andlithography steps and an anisotropic RIE to form a mask (FIG. 4a) whichdefines the structure to be fabricated; performing an upper levelvertical Si-RIE (FIG. 4b); conformally thermally oxidizing the samplefollowed by an anisotropic oxide etchback (FIG. 4c); performing anisotropic undercut RIE (FIG. 4d); performing a lower level verticalSi-RIE (FIG. 4e); conformally thermally oxidizing the sample, followedby an anisotropic oxide etchback (FIG. 4f); performing a lower levelrelease RIE (FIG. 4g), followed by a long thermal oxidation (FIG. 4h); athick resist lithography and wet oxide etch (FIG. 4i); and metallization(FIG. 4j).

As illustrated in FIG. 4a, a thick silicon dioxide film 160, which maybe 1.5-4 μm thick, is formed on the top surface of a single crystalsilicon substrate 162 by thermal oxidation, or the film is deposited bychemical vapor deposition (CVD). This film is patterned using a resistlayer 164, photolithography, and CHF₃ RIE to form a pattern mask 166.

The sequence of the following four process steps is repeated for bothlevels of the illustrated two-level MEM structures. First, the siliconsubstrate 162 is etched in a first vertical RIE step (FIG. 4b), usingthe silicon dioxide and photoresist films 160 and 164 as an etch mask,to form trenches 168. Typical etch depths range from 10 to 20 μm.Second, a subsequent thermal oxidation or CVD deposition covers thesubstrate surface conformally with a thin (200-400 nm) silicon dioxidelayer 170 (FIG. 4c). Third, a second vertical RIE removes this film fromthe horizontal surfaces. The silicon dioxide 170 on the sidewalls 172 ofthe SCS structures exemplified by beam and wall structures 174 and 176,respectively, is not etched, but is removed from the floor 178 of thetrenches to expose the underlying substrate 162. The tops of the SCSstructures are still covered with the remainder of the first, thickersilicon dioxide etch mask 160. Fourth, a high-pressure (80-90 mTorr),nearly isotropic SF₆ RIE of silicon, indicated by arrows 180, thenundercuts the masked silicon structures at 182 and 184 (FIG. 4d),releasing beam 185 and completing the fabrication of the first level ofstructures, as generally indicated at 186. So far, the process followsthe sequence of the SCREAM process of FIGS. 1a-g.

The same four steps illustrated in FIGS. 4b-4 d are repeated for thesecond level, as illustrated in FIGS. 4e-4 g. Thus, the siliconsubstrate is etched in a third vertical Si-RIE, indicated by arrow 188,for a further 10-20 μm (FIG. 4e), as illustrated at trench extension190. The whole upper-level masked silicon structure 186 serves as a‘shadow mask’ for this etch so that the silicon underneath the maskedsilicon structure 186 is not etched due to the high degree of anisotropyof the Si-RIE 188. The resulting lower-level silicon structure 192 isself-aligned to the upper-level structure 186, but is slightly widerthan the original width of the upper level, since the sidewall oxidedeposition 170 has increased the width of the upper level structure.

The next step is another thermal oxidation or CVD deposition 200 (FIG.4f), which again covers all surfaces conformally with silicon dioxide.Next, a vertical CHF₃-RIE indicated by arrow 202 removes the oxide fromthe horizontal surfaces to expose the floors 204 of the trenches, butleaving the tops of the upper level silicon structures still covered bythe remaining part of the first (thick) mask oxide 160. A nearlyisotropic SF₆ RIE, illustrated by arrow 206, is used to undercut thelower masked silicon structures 192 (FIG. 4g) at 208 and 210, and torelease the narrow beam 212. This completes the fabrication of thesecond level 192 of the MEM structures. If more than two levels are tobe fabricated, this sequence (FIGS. 4e-g) is repeated for eachadditional level.

After the completion of all levels of suspended microstructures, a timedthermal oxidation step is used to electrically isolate the suspended SCSstructures by consuming all the silicon in thin isolation segments whichare designed as part of the MEM structure, converting the silicon toelectrically insulating silicon dioxide. This is illustrated in FIG. 4h,which, for convenience, illustrates additional released beams 185′ and212′, fabricated in the manner described above for beams 185 and 212,but designed to be thinner (narrower) in cross-section. The thermaloxidation produces a layer of silicon dioxide 220 on all exposed siliconand increases the thickness of the oxide where it had previously beendeposited. As illustrated, the beam 185′ is designed to have at least aportion which is sufficiently thin that all of the silicon is consumed,leaving the thin portion of the beam formed completely of silicondioxide, and thus electrically insulating.

FIG. 5 illustrates an enlarged cross-sectional view of a MEM structurefabricated in accordance with the two-level process of FIGS. 4a-4 h. Thestructure 222 includes upper and lower beams 223, 223′; 224, 224′; 225,225′ and 226, 226′ which were fabricated in the silicon substrate tohave different nominal (CAD) widths of 4.0 μm; 2.0 μm; 0.8 μm and 0.6μm, respectively. As described above, the upper level beams 223, 224,225 and 226 were etched using a vertical silicon RIE, followed by athermal oxidation step or by a CVD oxide deposition step. The resultingupper beams, widened by the oxidation step, form a mask for the verticalsilicon RIE which defines the lower level beams 223′, 224′, 225′ and226′. Both levels are again oxidized, as indicated by oxide layer 228and the lower level is released. A long oxidation step consumes thesilicon in the thinnest beams.

In one example of the process, the initial oxidation of the upper levelbeams may increase the width of the upper beam by 200 nm, and thisproduces a lower level beam 200 nm wider than the corresponding upperlevel beam. Thus, a 0.6 μm-wide beam 226 on the upper level shadows a0.8 μm-wide beam 226′, a 0.8 μm-wide beam 225 shadows a 1.0 μm-wide beam225′, and so on. In this example, the final thermal oxide step may beselected to consume a total of 0.8 μm of silicon beam width, producingthe structure illustrated in FIG. 5. The oxide extending through thecomplete width of beams 225, 226 and 226′ produces an electricallyinsulating barrier in that beam segment, while retaining the mechanicalintegrity of the beam.

To enable the MEM device to be electrically connected to externalcircuitry, or to circuitry fabricated on the same wafer, themicrostructures are covered with a thick photoresist film 230, typically20-60 μm thick, as illustrated in phantom in FIG. 4i, and a secondlithography step is used to open contact windows such as window 232 onthe suspended structure and window 236 on the wall portion. Thesewindows are designed to be large enough (>20 μm features) to be easilyexposed in the thick resist and to not require critical alignment to theMEM beams 185, 185′, 212, and 212′ (which may be 0.6 to 4 μm wide), aswell as the wall portion formed by the surrounding substrate 62.Moreover, the windows do not have to be exposed through the fullthickness of the resist but only down to the tops of the upper levelbeams 185 and 185′, and the top of the wall portion when contact is tobe made to these features. The resist 230 forms an etch mask for a wetbuffered oxide etch which will open up the contact windows to expose theunderlying silicon.

As illustrated at window 232 in FIG. 4i, the oxide etch can also be usedto selectively remove the SiO₂ films from selected structures, such asthe beam 212′, and to remove a completely oxidized structure such asbeam 185′. In this case, the openings in the resist should be largeenough that they are exposed all the way to the bottom of the suspendedstructures.

After stripping the resist 230, metal contacts 236, 238, and 240 areapplied to the suspended silicon structures 212′, 185, and wall portion162, respectively, by the evaporation of aluminum, followed by asintering step (FIG. 4j).

Another embodiment of the invention utilizes the combined etch based onSCREAM and the Laermer et al process for the first level of a multilevelMEM structure, as described with respect to FIGS. 3a-3 e. These stepsreplace the steps illustrated in FIGS. 4a to 4 d, described above, withthe resulting structure of FIG. 3e serving as a shadow mask to fabricatethe next lower level, following the process steps of FIGS. 4e-4 g. Inthis multiple-level process, a sidewall silicon dioxide is stillrequired to achieve the broadening of the suspended structures on lowerlevels that is essential to the isolation and contact scheme. Additionallayers may be fabricated using the combined etch, but due to itslimitations discussed above they are commonly fabricated by repeatingthe steps of FIGS. 4e-4 g.

After completion of all of the desired levels of suspended structures,the timed thermal oxidation of FIG. 4h is used to electrically isolatethe suspended silicon structures by consuming all the silicon inselected thin isolation segments on all levels. Thereafter, thestructures are covered with a thick photoresist film (typically 20-60mm) and a second lithography step is used to open contact windows on thesuspended structures as described with respect to FIG. 4h. These windowsare designed to be large enough (>20 mm features) to be easily exposedin the thick resist and to not require critical alignment to the MEMbeams (which are 0.6 to 4 mm wide). Moreover, they do not have to beexposed through the full thickness of the resist but only down to thetop of the upper level beams. The resist forms an etch mask for a wetbuffered oxide etch to open up the contact windows. This etch may alsobe used to selectively remove the SiO₂ films from part of the suspendedstructures (FIG. 4i). In this case the openings in the resist have to belarge enough such that they are exposed all the way to the bottom of thesuspended structures. After stripping the resist contacts, the suspendedsilicon structures are formed by the evaporation of aluminum, followedby a sintering step (FIG. 4j).

A successful isolation and contact scheme for multiple-levelself-aligned structures such as those fabricated by the foregoingprocesses has to be able to selectively contact and isolate structuresindividually as well as all of them simultaneously, while maintainingmechanical support of the suspended structures. Furthermore, such ascheme has to provide a way to electrically connect to the suspendedlevels from the outside via contact pads at the wafer surface for alllevels. An example of a combination of beams and beam segments used toelectrically interconnect the various parts of a two-level structure areillustrated in FIGS. 6a and 6 b. Thus, a two-level structure 250comprises an upper level 252 made up of interconnected longitudinalbeams 254 and cross beams 256, and a lower level 258 made up ofinterconnected longitudinal beams 260 and cross beams 262. The upper andlower beams are vertically aligned by the fabrication process describedabove, with the beams 260 and 262 on the lower level being wider thanthe corresponding beams 254 and 256 in the upper level. In thisillustrated embodiment, the silicon core of the upper beams may be 2 mmwide. The beams are electrically conductive, but are coated with anoxide layer 264 in the manner discussed above, so as to be electricallyisolated. The upper and lower levels are mechanically isolated from eachother, but are mechanically supported on the substrate 266 by way offully oxidized, narrow support beams 268, which may be 0.6 mm thick, forexample.

To provide an electrical connection between the structure 250 and acontact 270 on the substrate 266, the silicon dioxide layer 264 isselectively removed from part of a beam, such as at longitudinal beamsegment 272 and cross beam segment 274 (FIG. 6a) on the upper level 252.Thereafter, an aluminum film 276 may be evaporated on the top of theupper level beams or beam segments which have been exposed by selectiveremoval of the oxide. This Al film also forms the top layer on thecontact pad 270, which is used to connect the beams to externalcircuitry. The dotted line 280 illustrates the current path from thecontact pad to the suspended beams.

An example of a structure for connecting the lower level 254 of thetwo-level structure 250 to contact pad 270 on substrate 266 isillustrated in FIG. 7, wherein the beams are fabricated as describedwith respect to FIG. 6. The structural beams 254 and 256 are generally 2mm wide, to provide the isolation between levels as described. However,in this case selected beam segments, such as cross beam segments 280,282 are designed to be wider than the rest of the beams, for example 4mm wide for the upper beam segment 280, so that during the release stepin the fabrication process, the beam segment 280 is not completelyundercut, and thus is not released from the underlying segment 282. Thiscreates a bridge 284 between the upper and lower levels, to allowcurrent flow through the Al layer 276, beam segment 280, bridge 284 andbeam segment 282 to the lower level beams 260 and 262, as indicated bycurrent path 290.

In order to avoid short circuiting the upper and lower levels, thesegments 280 are isolated by 0.8 μm wide segments 292, which are fullyoxidized on the upper level 252 but are thick enough to maintainelectrical contact on the lower level. The Al layer 276 on top of thecontact pad 270 is connected to the part of the upper level which isconnected to the lower level by opening up a contact window 294 in thesilicon dioxide covering the beam segment 280.

FIG. 7b is an electron micrograph of an actual structure, and shows howthe window 294 for the wet silicon dioxide etch is wide enough toinclude 0.8 mm wide isolation segments in region 296. This is crucial toobtain isolation between the suspended levels without a furtherlithography step, for if the SiO₂ beam segments were not removed, the Alfilm on top of the upper level structure would run from the lower levelcontact pad across the suspended structure to the upper level contactpad. This short-circuit between the levels can be removed by anadditional lithography step followed by an Al etch, but is eliminatedmore conveniently by simply removing the SiO₂ segments 296 on the upperlevel when the contact windows are etched before Al deposition. Usingthis solution the lower level contact structure does not providemechanical support for the upper level, while the upper level contactstructure provides mechanical support for both levels.

This isolation and contact concept can be extended to more than twolevels: for three levels it requires 6 different beam segment featuresfor (I) providing electrical isolation and mechanical support from thesubstrate for all three levels and isolation of suspended elements onall levels; (ii)isolation on the top and middle levels, but connectionon the bottom level; (iii) isolation on the top level, but connection onthe middle and bottom levels; (iv) separate levels without mechanical orelectrical contact; (v) electrical connections between the top andmiddle levels; and (vi) electrical connections between all three levels.

FIG. 8 shows diagrams of these six different elements used in thethree-level isolation scheme generally indicated at 300. The nominal CADbeam widths vary from 0.6 μm to 6 μm, as illustrated by beams 302-307.The fabrication sequence is similar to those described above; after thevertical Si RIE of the top level 310, a thermal oxidation step widensthe underlying structures in level 312 by 0.2 μm. The wider middle level312 is etched by vertical RIE and widened again in the sidewall thermaloxidation by another 0.2 μm. Thus the bottom level 314 structures are0.4 μm wider than the line width of corresponding top-level structures.The final long oxidation release step is timed to lead to the completeoxidation of 1 μm wide beam segments. Thus, the nominal 0.6 μm widebeams 302, which are 0.6 μm, 0.8 μm and 1.0 μm wide on the top, middleand bottom levels respectively, are fully oxidized on all levels toprovide the electrical isolation of feature (I). The nominal 0.8 μmbeams 303 are fully oxidized on the top (0.8 μm wide) and middle levels(1.0 μm wide) but maintain electrical contact on the bottom level (1.2μm wide)to provide feature (ii). The nominal 1.0 μm wide segments 304are only fully oxidized on the top level but electrically conducting onboth the middle (1.2 μm wide) and bottom (1.4 μm wide) levels to providefeature(iii). The nominal 2.0 μm and wider levels are conducting on alllevels but their inter-level connections differ: the 2.0 μm beams 305are electrically isolated from each other to provide feature (iv), the4.0 μm wide segments 306 provide an electrical connection between thetop and middle levels while maintaining isolation from the bottom levelfor feature (v), and the 6.0 μm wide pieces 307 electrically connect allthree levels for feature(vi).

The contact structures to connect three individual levels 320, 322 and324 to the top of a contact pad are illustrated in FIG. 9. FIG. 9a showsa top level contact structure 318 which is equivalent to the structureof the two level case of FIG. 6a, wherein 2 μm wide structural beamsprovide isolation between all levels and are isolated on all levels fromthe substrate by nominal (CAD) 0.6 μm wide beam segments which providethe mechanical support, as described above. Contact windows 326 on thetop-level structure beams 320 make electrical contact to the Al film 328covering the contact pads and the top-level structures 320.

FIG. 9b illustrates a structure 330 to electrically connect a contactpad to the middle level 322, which is similar to the lower level contactstructure of the two-level process shown in FIG. 7. All levels areisolated from the substrate by 0.6 μm wide beams, and 4.0 μm widesegments contact part of the top level 320 to the middle level 322, butnot to the bottom Level 324. The part of the top level 320 which isconnected to the middle level by the 4.0 μm segments is isolated fromthe rest of the top level by 1.0 μm wide beams. The 1.0 μm segmentsinsulate the top level 320 while providing electrical connections to themiddle level. The bottom level is mechanically connected to thesubstrate by the insulating 1.0 μm beams (the widened third level of thenominal 0.6 μm beams)

FIG. 9c shows a bottom level contact structure 340. In this level, theinter-level connection segment 342 is now 6 μm wide, connecting allthree suspended levels. The isolation segments to the left of theinter-level contact are nominally (CAD-linewidth) 0.8 μm wide, isolatingthe top and middle level from the interconnected levels. Thus, only thebottom level 324 is connected to this section, which is connected inturn to the Al film 328 on the contact pad through a contact window onthe top.

The isolation concept presented here can be generalized to any number ofsuspended levels and requires only a single lithography step and onemetal deposition, regardless of the number of levels. However, thedegree of complexity of the structures which need to be fabricatedincreases rapidly: N levels of suspended structures require 2 Ndifferent widths of beam segments for the isolation and contact scheme.The thermal silicon dioxide bridges provide high quality insulationbetween the suspended levels and the substrate with low leakage currentsand high breakdown voltages, but the isolation scheme requires thedifferences in widths between the various levels to be tightlycontrolled.

The undercut RIE has to be accurately controlled as well to insure thedesired contacting and isolation from level to level. This etch isharder to control than the vertical RIE angle or silicon dioxidethickness, but the process window is also ten times larger because ofthe greater difference in beam width.

The multiple-level process of the invention, as described above, allowsthe fabrication of multiple levels of self-aligned, high aspect ratio(HAR) single crystal silicon (SCS) structures, but there are limitationsto design freedom on the different suspended levels, for the lateralgeometry on each level has to be identical. Moreover, the thick silicondioxide film covering parts of the levels leads to stress problems,which may have an adverse effect on the mechanical properties andstability of the suspended structures. In accordance with furtheraspects of the invention, three variations and extensions of the basicprocess, which address these limitations, are available.

The simplest way to allow variation of the lateral geometry on differentlevels of a MEM structure can be accomplished by intelligent design anddoes not require any different or additional processing. It isillustrated in FIG. 10 by a two-level example 350, wherein a thin(nominal 0.8 μm wide) beam segment 352 is fully oxidized on the upperlevel 354, but not on the lower level 356. Thus when the structures areexposed to a wet oxide etch the upper level oxide beam 352 is removedwhile the silicon core 358 of the lower level remains, as illustrated inFIG. 11. This method can be used to generate features on the lower levelwhich do not exist on the upper level, and can also fabricate devicescontaining structures of different heights. For example, thin (nominal0.8 μm) beams would exist only on the lower level, while very wide(nominal 4 μm) wide beams would exist on both levels and would be twiceas tall. These structures can be used, for example, in micromachinedelectrostatic lenses and actuators. The same concept is extensible tomore than two levels, removing any number of levels from the top of thestack, as illustrated at 360 in FIGS. 12 and 13, wherein the thicknessesfor beams 362, 364 and 366 in level 368 and their correspondingunderlying beams in levels 370 and 372 are selected to permit removal ofbeams on various layers.

The approach of removing parts of the upper or top level(s) by fullyoxidizing and then dissolving them in a wet etch is very simple, but itcan only be used to remove parts of the upper level of a two levelstructure, or the top or the top and middle levels of a three levelstructure. The lower or bottom level can never be modified in thismanner. In addition, the width of the beams which can remain on thelower level is limited to the amount of broadening between levels. Asecond, more flexible approach to introducing variations betweenstructural levels uses a focused ion beam (FIB), illustrated at 380 inFIGS. 14a and 14 b for an upper level MEM beam 382 and in FIGS. 15a and15 b for a lower level MEM beam 384. The MEM beams 382 and 384 areindividually modified after the beam fabrication sequence describedabove has been completed; sections of different levels then are removedselectively by milling or cutting with the FIB 380. Tilting the samplerelative to the incident beam allows the removal of the lower levelwithout affecting the upper level, as illustrated in FIG. 15a and 15 b.The FIB modification approach to varying the geometry between levels ismuch more flexible than the wet etch approach described above; itremoves the limit on the linewidth of structures which only exist on thelower level, since as any width of upper level structure can be removedarbitrarily. It further allows the selective removal of sections of thelower level and thus makes it possible to fabricate devices withelements which exist on the upper but not the lower level.

The removal of lower level elements requires tilting the sample so thatthe ion beam 380 can strike the lower level structure 384, withoutdamaging the upper level structure 382 above it. As illustrated in FIG.16, this requires a large enough spacing between adjacent structures topermit the ion beam to reach the structure to be removed, with the limitbeing determined by the incident angle alpha and the combined height ofthe suspended levels 382 and 384 and inter-level gaps 386; i.e., thetotal height of the structures h1+h2+g. This removal technique can notbe applied to closely spaced structures such as actuator electrodes.

A third extension of the basic multiple-level process uses selectiveoxidation to introduce variations between the suspended levels and toreduce the thickness of the sidewall silicon dioxide films. A siliconnitride mask is deposited conformally on a suspended SCS structure andis then patterned with a thick-resist lithography step and etched in hotphosphoric acid. The remaining silicon nitride acts as a mask during thesubsequent thermal oxidation step. This method can be appliedselectively to each level. The feature size of the oxidized or maskedelements is limited by the resolution of the thick-resist lithographystep.

A variety of multiple-level (ML) actuators may be fabricated using theforegoing processes. Such actuators consist of multiple sets of fixedand moving beams which may act as MEM device electrodes, which areelectrically isolated from each other. The movable ML actuatorelectrodes typically are comb-type electrodes attached to a singlemoving structure which may be up to several levels tall. Variations inthe number of levels and the heights of the electrodes allows a numberof different actuator designs. FIGS. 17a to 17 e illustratecross-section diagrams of four actuator electrode configurations for atwo-level device. FIG. 17a illustrates a comb-type electrode showing twolevels of moving electrodes at the center, surrounded on either side bytwo levels of fixed electrodes; FIG. 17b illustrates two levels 404 and406 of fixed electrodes, and a single tall electrode 408; FIG. 17cillustrates two tall fixed electrodes 410 and 412 and a short movingelectrode 414; and FIG. 17d illustrates short fixed electrodes 416 and418 and a tall moving electrode 420. FIG. 17e illustrates aconventional, single-level comb actuator 422 having moving and fixedelectrodes and which is included for comparison. All these designs maybe fabricated using the process of the invention.

The multiple-level (ML) actuators consist of two levels of isolatedelectrodes, and one (FIG. 17b) or two (FIG. 17a) sets of movingelectrodes. Multiple-height (MH) actuators contain only one set ofmoving and fixed electrodes, but the two sets of electrodes differ inheight, preferably by more than a factor of two. For actuators composedof three or more levels of electrodes combinations of ML and MH designsare also possible.

The diagrams in FIG. 17 are scale-drawings of fabricated structureswhich are examples of these devices. In this example, the lateralspacing between the fingers is 5 μm, as is the vertical spacing betweenthe two suspended levels. The distance from the lower level to thesubstrate floor, indicated at 424, is varied from 10 to 20 μm. The widthand height of the electrodes vary depending on the type of electrode,with the regular electrodes, existing on both levels but not connectedto each other as in FIGS. 17c and 17 d, are 1 μm wide and 15 μm tall.The tall electrodes, which span two levels, are 35 μm tall and 3 μmwide. Other sizes and configurations are possible in accordance with theinvention.

The ML and MH actuators are designed for out-of-plane motion(perpendicular to a wafer surface). Conventional comb-actuators havebeen shown to generate a vertical force component due to the asymmetryintroduced by the substrate below the comb fingers, but theherein-disclosed actuator designs generate a larger vertical force overa greater stroke than the conventional comb drives of comparabledimensions. Furthermore, the ML actuators are bi-directional.

The force vs. displacement characteristics of all the actuator designsin FIG. 17 have been studied by numerical simulation. The simulationresults are given per unit length [μm] of a single moving electrode. Theapplied bias is 0V or 1V, and the displacement step between thesimulated configurations is 1 μm. The calculated forces have beennormalized by a factor F₀, related to the force F_(p) generated by aconventional comb-actuator parallel to the surface of a wafer, which is$F_{p} = \frac{ɛ_{0}{hV}^{2}}{g}$

Here V is the applied voltage, h the height of the actuator, g the gapbetween the electrodes and e₀ is the permittivity of free space. Thescaling factor is this force, per unit height of the actuator electrodeat 1V applied bias:$F_{0} = {{100\frac{ɛ_{0}}{g}} = {1.77 \cdot {10^{- 4}\left\lbrack \frac{\mu \quad N}{{\mu {mV}}^{2}} \right\rbrack}}}$

for an electrode gap of 5 im. Thus the vertical axis of the plots in thefollowing Figures gives the force generated by the actuatorconfiguration (per unit length of electrode) as a percentage of that ofthe in-plane force of a single-level comb actuator with the same gapwidth (per unit electrode height).

The multiple-height (MH) actuators are the out-of-plane or z-actuatorsthat most resemble conventional comb actuators. They contain only oneset of moving and fixed electrodes each, but the vertical asymmetry andthus the levitation or pull-in force generated is increased relative tothat of conventional comb actuators; the height of the fixed and movingelectrodes differs preferably by more than a factor of two. As theycontain only one set of moving and fixed electrodes, these actuators donot necessarily require a multiple-level contact and isolation schemeand can thus also be implemented using a simplified process.

The electrode configurations shown in FIGS. 17c and 17 d have beensimulated for the two bias configurations of the moving electrodes;namely, grounded and biased. The simulation results for the situationwhere the moving electrodes are grounded are shown in FIG. 18. This isthe most commonly used bias configuration in conventional MEMS for tworeasons: it does not require isolation of the moving structure from thesubstrate, and it does not give rise to a ‘parasitic’ actuation forcebetween the biased suspended structure and the grounded substrate.However, neither concern is relevant for devices fabricated by theprocess of the present invention, as it allows for isolation from thesubstrate and within the suspended structures. Thus, the movingelectrodes can be biased while the moving stage is grounded.

FIG. 18 shows the force vs. displacement of the two MH actuator designsand a comparable conventional comb actuator over the full stroke ofthese actuators. The force varies with displacement for all threeactuator designs, unlike for the in-plane comb-drive. The z-forcegenerated by a conventional comb-drive (FIG. 17e) is essentially arestoring force, varying linearly near the equilibrium position (z=0).It is offset from z=0 due to the asymmetry introduced by the groundplane. For large displacements (dz>10 μm) it approaches a constantlevel.

The design of FIG. 17c creates a larger levitation force up to adisplacement of approximately dz=10 μm. At z=10 μm the short movingelectrode 414 is centered vertically between the tall fixed electrodes410 and 412. The position where the force is zero is shifted slightly toa larger z-value by the presence of the substrate ground-plane 424 belowthe actuator electrodes. As z increases further the force is nowdownward (pull-in). At z=20 μm, the top of the moving and fixedelectrodes are aligned, mirroring the configuration at z=0. Themagnitude of the force generated (at z=20 μm) is reduced (from to thevalue at z=0), as the levitation caused by the substrate counteracts theforce generated by the fixed comb fingers.

The design of FIG. 17d generates a downward, or pull-in, force over theentire plotted range. It vanishes as z approaches −10 μm, at which pointthe moving electrode is centered between the fixed electrodes. Themaximum force is generated near z=10 μm, where the overlap between themoving and fixed electrodes is 5 μm, the same as the gap width. As zincreases further the generated force drops. This is the same behavioras that observed for in-plane comb actuators as the comb-finger approachand begin to overlap.

An expanded view of the region around z=0 is shown in FIG. 19. This isthe region that is generally most relevant to actual operation. Over theplotted range (−5 μm<z<5 μm), both MH actuator designs generate largerout-of-plane forces than the conventional comb actuator. Furthermore,the direction of the force they generate does not change, unlike that ofthe conventional comb actuator.

In most MEMS devices, as exemplified by the device of FIG. 2, theactuators (50, 56) are used to move structures 40 suspended from thesubstrate by a set of springs 44, 46. These springs exercise a restoringforce towards the equilibrium position that the actuator has to overcomein order to deflect the moving stage, i.e. the generated force has to beupward (F>0) for levitation (z>0) and downward (F<0) for pull-in (z<0).Thus there is a useful range of actuation of a motor for the z-valuesover which it meets the two above criteria. Applying this criterium tothe three actuator designs shown in FIGS. 17c, d and e, the useful rangeof motion for the MH actuators (0 to 5 μm for levitation, 0 to −5 μm forpull-in) is found to be ten times larger than that of a conventionalsingle-level comb-drive with the same dimensions (design of FIG. 17e) (0to 0.44 μm, levitation only). The force generated by the MH actuatorsover this range is more than five times larger than that of theconventional comb-actuator. For resonant operation the direction of theforce generated should, moreover, not change as the electrodes passthrough the equilibrium position. This requirement is met by all threeactuator designs.

The second bias configuration for the MH actuators is to bias the movingelectrodes and ground the fixed electrodes. The resulting force vs.displacement characteristics are plotted in FIG. 20. At z>0 the behavioris similar to that shown in FIG. 18 for the moving electrode grounded.As z decreases below −5 μm, however, the force is dominated by thepull-in effect of the grounded substrate on the biased moving electrode.The conventional comb actuator (FIG. 17e) now generates a pull-in forcefor z>−1.23 μm. The forces generated by the MH actuators have notchanged qualitatively; the design of FIG. 17c still generates levitationat z less than approximately 10 μm and pull-in above, but the magnitudesof the forces have changed due to the pull-in effect exerted by thesubstrate on the biased moving electrode. The design of FIG. 17dgenerates a pull-in force over the entire plotted range at an increasedmagnitude compared to the configuration with the moving electrodesgrounded.

The out-of-plane forces generated by these comb actuator designs dependon the vertical asymmetry of the electrode arrangement. In the case ofthe conventional comb actuator this asymmetry arises entirely from thepresence of the grounded substrate floor. The above simulations showthat the substrate floor also has a discernible effect on forcesgenerated in the MH actuator design. This effect is illustrated in FIG.21, which shows the dependence of the force generated at z=0 on thespacing between the bottom of the moving electrodes and the substratefloor (with the moving electrodes grounded).

For the conventional comb actuator the generated force decreases to zeroas the substrate is removed. For the MH levitation actuator of FIG. 17cthe force decreases as well, but levels off at approximately 40% of thein-plane comb drive value (F₀). For the pull-in MH actuator of FIG. 17d,the force increases with the substrate-to-structure separation andlevels off near 25% of F₀.

The characterization of the ML actuators is more involved: there aremore than two sets of electrodes allowing the simultaneous applicationof several voltages. This leads to a large number of bias configurationsand force vs. displacement characteristics. The generated force dependson the square of the applied voltage-differences and this can not besolved by the superposition of multiple simulation results. There are,however, two essential characteristics of the ML actuators that can notbe achieved with conventional electrostatic actuators: first, theyillustrate the bi-directional operation of a ML actuator, and secondthey form of a bi-stable system in conjunction with a restoring springsuspension.

The same ML actuator can generate either a levitation- or apull-in-force over the same range of z-positions for different electrodebias configurations. This is illustrated in FIGS. 22 and 23 for atwo-level actuator of the design of FIG. 17b. FIG. 22 shows the biasconfigurations for the levitation and pull-in modes, and FIG. 23 showsthe force vs. displacement for the range of −10 μm<z<10 μm.

In the levitation mode (solid line in FIG. 23) the upper fixedelectrodes are biased while all the other electrodes are grounded. Inthe pull-in mode (dashed line in FIG. 23) the lower fixed electrodes arebiased while all other electrodes are grounded. The generated force(both levitation and pull-in) does not change direction over the entireplotted range and decreases to zero at a z-displacement of approximately10 μm. At this point the tall moving electrode is vertically centeredbetween the biased fixed electrodes. These characteristics are similarto the MH drives discussed above, but the generated force isapproximately 50% lower.

The force vs. displacement relationship for a two-level actuator of thedesign of FIG. 17a is shown in FIGS. 24 and 25 for both levitation andpull-in configurations. The useful range of actuation of this actuatoris much smaller than that of the design of FIG. 17b; the generated forcedrops more rapidly as |z| increases and the force changes direction atapproximately 2 μm. This effect occurs because the biased fixedelectrodes simultaneously exert opposing forces on the two levels ofmoving electrodes: e.g. in the levitation mode (upper fixed electrodesbiased, all others grounded), the upper moving electrode (displacedabove the biased fixed electrodes) is pulled downward while the lowermoving electrode (below the biased fixed electrodes) is pulled upward.This effect is strongly dependent on the vertical gap between theelectrode levels and disappears as this gap vanishes.

In order to facilitate comparison of the actuator designs the levitationmode characteristics have been plotted in FIG. 26 for the ML designs ofFIGS. 17a and 17 b, and the conventional comb actuator. The forcegenerated by the ML actuators exceeds that of the conventional combactuator over the entire plotted range. The useful range of actuation isincreased by a factor of 4 and 12 for designs (a) and (b), respectively.Over this range the ML actuator force exceeds that of the conventionalcomb actuator a factor of 2.5 and more.

The force vs displacement characteristic of bi-directional actuation ofa multiple level actuators such as in FIG. 17(b) is shown in FIG. 27.The plot of the pull-in curve has been inverted through the origin toillustrate its similarity in the force vs. displacement characteristicto the levitation mode. The two traces are nearly identical, with thepull-in mode slightly smaller in magnitude. This reduction is due to thelevitation effect of the substrate below the comb electrodes, as wasdiscussed above for the MH actuators.

Besides operating as a bidirectional actuator the ML design of FIG. 17acan also be used to create a bi-stable system. FIG. 28(a) illustratesthe bias configuration: the upper moving electrode 440 and the lowerfixed electrodes 442 and 444 are biased, all other electrodes aregrounded. The upper fixed electrodes 446 and 448 exert an upward forceon the lower moving electrode 450 and the lower fixed electrodes 442 and444 pull downward on the upper moving electrode 440. At the equilibriumposition these forces balance. A small out-of-plane perturbation ineither direction causes the force in the same direction to increase andthe opposing force to decrease, which leads to a further increase indeflection. The resulting force vs. displacement curve is plotted inFIG. 28b.

The combination of this actuator configuration with a restoring springsuspension allows the formation of a bi-stable system. The spring adds alinear restoring force. The potential energy of the combined system fora number of applied voltages is shown in FIG. 29. When no force isapplied (V=0) the potential energy trace is a parabola, corresponding tothe spring suspension. As the applied voltage (and thus force) isincreased, this combined parabola broadens—the actuator acts initiallyas a spring with a negative spring constant. As the voltage is increasedfurther it eventually leads to the formation of a double-well potential.

A novel clamp-alignment structure 460 fabricated by the process of theinvention is illustrated in FIGS. 30a and 30 b. This is a multiple-leveldevice that consists of two identical upper and lower stages 460 and 462with diamond-shaped openings 464 and 466, respectively at the center.The stages are anchored to the substrate by a conventional folded-beamsuspension (not shown) and are displaced in the plane of the wafer bycomb actuators 468 and 470. At no applied force (0V) the two stages areself-aligned to each other. Applying a voltage to either actuatortranslates the corresponding stages to the left or right, decreasing thesize but maintaining the shape of the overlap region 472 between thediamond-shaped openings in the stages. When an external element 474 suchas an optical fiber or a post on top of a second chip is inserted intothe stage opening 472, it is aligned in the y-direction by the relativeoffset of the diamond shapes, and in the x-direction by the displacementof the moving stages.

A simpler, but more limited version can also be implemented using amoving top and a stationary bottom level. In this case the upper leveldiamond-shaped aperture is used to push the external element acrossuntil it rests against the fixed lower level diamond aperture.

The diagonal of the diamond-shaped openings 464 and 466 determines themaximum diameter object 474 which can be clamped and aligned in thismanner (d_(max)=0.71F). The minimum-diameter object that can be alignedthen depends on the range of motion Δx_(max) of the comb actuators(d_(min)=F−2ΔX_(max)). Optical fibers and supports of microfabricatedstructures are likely candidates for alignment using this structure.Typical values for their diameters range from 50 μm to 200 μm. Thistranslates into a large aperture diameter (100-300 μm) and largerequired stage displacement (25-100 μm).

The suspension stiffness must be linear for large displacements and verycompliant in the direction of the desired displacement to minimize therequired actuation voltage, and at the same time very stiff in theperpendicular directions to avoid misalignment and in particular failureof the comb actuators.

This leads to the following possible design choices for thefolded-spring suspension system used. The spring beams are made verylong to minimize the non-linear effects at large deflections. Theirwidth is limited by the process to less than 3

μm. This requires the use of several springs in parallel to achieve thedesired stiffness. Selective increases in the stiffness in thedirections perpendicular to the desired displacement are limited. k_(z)increases with the height of the structures. The ratio of the in-planespring constants k_(x)/k_(y) scales with the square of the ratio w/l;thus, increasing the length of the beams and the number of beams inparallel to maintain k_(x) increases this ratio. The suspension designparameters for three different aperture sizes are summarized in Table 1.

TABLE 1 Design parameters for clamp structure suspension (all lengths inμm, stiffness in N/m) F W₁ w₂ l k_(x) k_(y) k_(z) 40 1.5 250 1.64 410072.9 40 1.7 250 2.39 5968 82.7 100 1.5 300 0.95 3420 42.2 100 1.7 3001.38 4979 47.8 100/200 1.5 350 0.60 2940 26.7 100/200 1.7 350 0.87 428030.1 200 1.5 400 0.40 2560 17.5 200 1.7 400 0.58 3712 20.1

The actuators driving the stages need to provide a large force andoperate over a long range of motion. Comb actuators meet these designcriteria. The high force requires a large number of combs and a smallgap between the fixed and moving electrodes. The long range of motionrequires long electrode fingers. This raises the issue of stability ofthese actuators, which is illustrated in FIG. 31.

The lateral force in a comb actuator is zero at the center of the gap(FIG. 31a) but for slight displacements in y—perpendicular to thefingers (FIG. 31b)—it increases rapidly. The lateral force FL is givenby${F_{L}(y)} = {2ɛ_{0}V^{2}{h\left( {x_{0} + {\Delta \quad x}} \right)}\frac{gy}{\left( {g^{2} + y^{2}} \right)^{2}}}$

where V is the voltage applied to the actuator ₁x₀ the initial electrodeoverlap at zero displacement, Δx the displacement in x, and g the gapbetween the electrodes. This can also be expressed as a (negative)spring constant of the actuator k_(y,act)$h_{y,{act}} = {- \frac{2ɛ_{0}V^{2}{h\left( {x_{0} + {\Delta \quad x}} \right)}}{g^{3}}}$

This equation shows that the actuator gap has a much stronger effect onthe destabilization of the perpendicular mode than the increase ingenerated force. Thus in order to obtain the required force it is betterto increase the number of fingers than reduce the gap. The actuatordesign parameters chosen for three different aperture size stages anddisplacement ranges are summarized in Table 2.

TABLE 2 Design parameters for clamp structure actuators N g F(V = 30V)F(V = 70V) 200 5 3.2 8.8 400 5 63.4 17.6 400 8 4 11 600 8 6 16.5 600 104.8 13.2 800 10 6.4 17.6

Although the invention has been described in terms of preferredembodiments, it will be understood that its true spirit and scope islimited only by the following claims.

What is claimed is:
 1. A microelectromechanical structure comprising: asubstrate having a cavity formed by etching through a single mask toproduce a two-level suspended structure in said substrate, the structureincluding: a first level incorporating a first clamp stage suspended insaid substrate and movable by a first set of actuators; a second levelincorporating a second clamp stage suspended in said substrate andmovable independently of said first stage by a second set of actuators,said first and second levels being substantially identical andvertically self-aligned, said first and second sets of actuators beingoperable in opposite directions to shift one of said first and secondclamp stages with respect to the other.
 2. The structure of claim 1,wherein said substrate lies in a horizontal plane and said first andsecond clamp stages are movable in planes parallel to each other and tothe plane of said substrate.
 3. The structure of claim 2, wherein saidfirst and second sets of actuators are comb actuators, and wherein atleast one of said sets of actuators is operable to cause one of saidstages to move horizontally with respect to the other of said stages. 4.The structure of claim 3, wherein each of said stages incorporates anaperture, said apertures being vertically aligned when said sets ofactuators are at rest, the relative motion of said stages displacing oneaperture with respect to the other.
 5. The structure of claim 2, whereineach of said stages incorporates an aperture, said aperture beingvertically aligned when said stages are at rest to provide an openingthrough said first and second stages, and wherein movement of one stagewith respect to the other decreases the size of said through opening. 6.A multilevel microelectromechanical structure, comprising: a firstvertical electrode having a first vertical height extending through afirst structure level; a second vertical electrode spaced horizontallyfrom said first electrode and having a second vertical height extendingthrough said first structure level and through a second structure levelto provide vertical assymetry, said electrodes being energizable toproduce relative vertical motion of one electrode with respect to theother.
 7. The structure of claim 6, wherein said height of said secondelectrode is at least twice the height of said first electrode.
 8. Thestructure of claim 6, wherein said first electrode is included in a setof spaced, parallel, electrodes each having said first height.
 9. Thestructure of claim 6, wherein said second electrode is included in a setof spaced parallel electrodes each having said second height.
 10. Thestructure of claim 6, wherein one of said electrodes is fixed and theother of said electrodes is movable.
 11. The structure of claim 6,wherein said first electrode is included in a first set of spaced,parallel electrodes each having said first height and said secondelectrode is included in a second set of spaced, parallel electrodeseach having said second height, said electrodes being interleaved toform a comb-type actuator.
 12. The structure of claims 11, furtherincluding a bias voltage selectively applied to said electrode sets tocause one set of electrodes to move vertically with respect to theother.
 13. The structure of claim 12, wherein one of said sets ofelectrodes is fixed and the other of said sets is vertically movable,whereby said sets provide a vertically displaceable actuator.
 14. Thestructure of claim 1, further including additional levels ofself-aligned, suspended structures.
 15. The structure of claim 1,wherein said self-aligned, suspended clamp stages are aligned to afeature on the substrate.
 16. The structure of claim 15, wherein thefeature on the substrate is an aperture.
 17. The structure of claim 6,further including additional suspended structures of varying heights.18. The structure of claim 6, wherein the electrodes, when energized,produce a vertical force which is greater than that achieved by acomparable structure employing electrodes of identical height.
 19. Thestructure of claim 6, wherein the electrodes, when energized, produce avertical force in a direction of displacement from an equilibriumposition, the range of displacement being greater than that achieved bya comparable structure employing electrodes of identical height.
 20. Amultilevel microelectromechanical structure, comprising: a first set ofelectrodes spaced horizontally and having a first vertical heightextending through a first structure level; a second set of electrodesself-aligned horizontally and spaced vertically from said first set ofelectrodes and having a second vertical height extending through saidsecond structure level; said second set of electrodes being mechanicallyattached to but electrically insulated from said first set ofelectrodes; both sets of electrodes being energizable to producerelative horizontal or vertical motion of one electrode with respect tothe other.
 21. The structure of claim 20, comprising three or more setsof electrodes, aligned to each other horizontally and spaced from eachother vertically.
 22. The structure of claim 20, wherein the electrodes,when electrically energized, produce a vertical force which is greaterthan that achieved by a comparable structure employing only a singlelevel of electrodes of the combined height.
 23. The structure of claim20, wherein the electrodes, when electrically energized, produce avertical force in a direction of displacement from the equilibriumposition over a greater range of displacement than that achieved by acomparable structure employing only a single level of electrodes of thecombined height.
 24. The structure of claim 20, wherein said sets ofelectrodes are energizable to produce a bidirectional vertical actuator,said electrodes being energized by a variation of electrode biasconditions.
 25. The structure in claim 20, further including means tobias said sets of electrodes to produce a multi-stable device withseveral stable equilibria determined by the biasing conditions of theelectrodes
 26. The device of claim 25, where the multi-stable device isproduced by applying bias voltages as follows: a first voltage isapplied in an inner electrode on an upper level and an outer electrodeon a lower level; and a second voltage is applied to an outer electrodeon the upper level and an inner electrode on the lower level.
 27. Amicroelectromechanical structure, comprising: a substrate having acavity formed by etching through a single mask to produce a multi-levelstructure; a first level released stage suspended in the cavity in saidsubstrate; a first set of actuators connected to said first level clampstage for moving said clamp stage; a second level clamp stage on saidsubstrate substantially identical to and vertically self-aligned withsaid first level clamp stage, said first and second level clamp stagesbeing relatively movable.
 28. The structure of claim 27, wherein saidstages lie in parallel planes.
 29. The structure of claim 28, whereinsaid stages incorporate apertures which are vertically aligned toprovide a through aperture when said stages are at rest, and whereinrelative movement of said stages along said parallel planes decreasesthe size of said through opening.
 30. The structure of claim 28, whereinsaid second level clamp stage is fixed to said substrate.
 31. Thestructure of claim 28, wherein said second level clamp stage is releasedand suspended in said cavity, and further including a second set ofactuators connected to move said second level clamp stage.